The impact of scanning probe microscopy (SPM) over the past 20 years has been dramatic: its invention was, for example recently rated the second most important advance in materials science of the past 50 years. SPM techniques share a common feature, the use of a probe to detect a spatially localized signal. In most cases it is the probe that limits the spatial resolution of the technique.
The most common form of SPM is atomic force microscopy (AFM), where the probe is a sharp tip, usually mounted on a microscale cantilever that acts to transduce the tip–sample force (the localized signal). A map of surface topography is constructed by scanning the tip across the substrate. The resultant image is a convolution of the tip geometry with the surface topography. Commercial AFMs can have sub-angstrom noise levels, but the realizable lateral spatial resolution is limited by the tip geometry and is typically around two orders of magnitude larger. For an ideal AFM tip, the exact tip geometry and chemistry should be known, the tip dimensions should be as small as possible without sacrificing rigidity, and the probe should be capable of imaging over a long lifetime, while aintaining a constant geometry.
Attempts have been made to create ‘ideal’ tips using methodologies such as focused ion-beam structuring of the tip apex. However, the above requirements are perhaps best met by carbon nanotubes: cylindrical shells of graphene with diameters as small as 1 nm. Indeed, carbon nanotubes show great promise as AFM tips despite the substantial challenges involved in their fabrication. [Wilson and Macpherson doi: 10.1038/nnano.2009.154] present a review article that looks at the progress in the production and application of carbon nanotube AFM tips since their arrival onthe SPM scene in 1996
A readily available source of nanotube tips would further open up the AFM imaging world, increasing tip longevity, reducing tip imaging artefacts, increasing resolution and decreasing tip–surface forces. It would also have a significant impact in key research areas such as structural biology, biotechnology, metrology and nanoelectronics.
The question then remains, why are nanotube tips not being used routinely for AFM imaging and characterization? The answer lies in the fabrication. Progress in this area is being made, but obstacles still remain. However, the significant rewards waiting will ensure that this remains an active area for the foreseeable future.